This invention relates generally to systems for dispersing, milling, or otherwise processing solid particles within a liquid and more particularly to an improved automatic control unit for remotely varying the speed of such a system.
Because of the large volume of product often being processed at one time in an industrial dispersing or milling operation, and because of the high viscosity of some of the product being dispersed, the rotational loads on the impeller or rotor and drive system are quite large. The start-up torque is particularly large if a system has been shut down for many hours in that the temperature of the product is usually lower at start-up and some liquids actually tend to gel. Thus, with a fixed-speed system, there is danger of overstressing the equipment, breaking components, etc. One advantage of a variable speed system is that the rotor can be started at a slow speed.
In one known variable speed system, an electric motor is employed to drive the rotor through a combination of V-belts and adjustable pulleys. In such an arrangement, the effective diameters of the drive pulleys are changed by axially varying the position of pulley halves and thereby varying the diameter of the pulley surfaces engaged by the drive belts.
There are a number of major disadvantages to such an arrangement. For example, the speed of the pulley combination can only be adjusted while the pulleys are being driven by the drive motor. In practice, an operator is instructed to return the pulley combination to a slow start-up speed prior to the end of a dispersion operation. However, if the operator neglects to take this step, a following dispersing operation must be initiated at a higher start-up speed than is desired. As a result, drive shafts and pulley belts can be damaged and the life of the drive motor reduced due to high resistance at the initiation of a processing operation which overloads the drive train of the system.
Typically, the pulley adjustment operation is accomplished by means of a control shaft and gear mechanism that is hand-driven by the operator. This control operation requires considerable force, is somewhat slow, and the control is often somewhat awkwardly positioned such that the operator is subjected to some danger of being too close to moving components in the system. These are undoubtedly some of the reasons that operators neglect to move the components to the slow speed position whenever the system is to be stopped.
Another drawback of this known system is that it typically requires an operator to give constant adjustment and attention to a single system.
Thus the need exists for improved controls for a variable speed slurry processing operation.